1. Field of the Invention
The present invention relates to a spot welding apparatus where the mechanical impedance of the apparatus before and during welding is controllable and a method for controlling the welding apparatus.
2. Description of Related Art
Japanese Patent Publication No. HEI 7-144283 discloses a method for controlling a pressing axis drive actuator of a spot welding apparatus. More particularly, the Publication discloses a method for determining a static pressing force and a feed back control for accurately controlling the static pressing force.
However, the conventional spot welding apparatus and the control method therefor have the following problems:
(a) Problems before Welding
In welding relatively thick workpieces to each other, it is difficult to ensure that the workpieces contact each other at the entire opposed surfaces thereof and a gap tends to form between the workpieces, since the workpieces have a relatively high rigidity. If a clamping mechanism contacts the workpieces at a low impedance, the workpieccs cannot contact each other. As a result, an optimum welding electrical current density and a sufficient contact stress are not obtained thereby degrading the welding quality.
In welding relatively thin workpieces to each other, a problem opposite to that of the above-described problem occurs. More particularly, since the thin workpieces have a low rigidity, if the contact impedance is not decreased, the contact area will be too large, which is accompanied by a great decrease in the welding electrical current density and detachment of the workpieccs at the weld.
(b) Problems During Welding
In thermal expansion and shrinkage of the workpieccs, the mechanical impedance greatly affects the welding conditions (pressing force, welding electrical current, and welding time period) and the welding quality.
More particularly, in the case of a low impedance of the welding clamp mechanism, though at an early stage of the spot welding the contact area is small and a sufficient welding electrical current density is obtained, at a nugget generating stage of the spot welding the expansion and shrinkage of the workpieces cannot be sufficiently restricted and the contact area tends to become too small. As a result, the welding electrical current density becomes too large and dispersion of the melted material tends to occur.
In the case of a high impedance, an increase in the pressing force due to thermal expansion of the workpicces occurs. As a result, at an early stage of spot welding, the contact area becomes large which is accompanied by a decrease in the welding electrical current density, a decrease in the welding strength, and detachment of the workpieces at the weld. In order to increase the welding strength, it is necessary to increase the welding electrical current and the capacity of the electrical source.